Crush biscuits in a food processor or with the end of a rolling pin until they resemble fine crumbs. Add melted butter and mix well to combine. Press biscuit mixture into the base and up sides of a 25cm springform cake tin. Leave about an inch to the top of the tin. Set aside.

Break chocolate into small pieces and place in a heatproof bowl. Stand bowl over a saucepan of simmering water and stir until chocolate has melted and is smooth. Remove chocolate from heat and set aside to cool.

Using an electric mixer, beat cream cheese and butter until smooth, gradually add caster sugar and mix well. Add melted chocolate and eggs and beat until just smooth. Don't overmix.

Pour cheese mixture over biscuit base. Bake for one hour. Turn oven off and allow cheesecake to cool in the oven with the door ajar for a further hour. Remove cheesecake from oven and allow it to stand on a wire rack until completely cool. Cover and refrigerate for at least four hours after that.